1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
The coverage area of a wireless communication system is typically divided into a number of cells, which may be grouped into one or more networks. Mobile units located in each cell may access the wireless communications system by establishing a wireless communication link, often referred to as an air interface, with a base station associated with the cell. The mobile units may include devices such as mobile telephones, personal data assistants, smart phones, Global Positioning System devices, wireless network interface cards, desktop or laptop computers, and the like. Mobile units may initiate communication with a base station by providing a signal on an access channel. The base station may then use the received access channel signal to establish the wireless communication link between the mobile unit and the base station on a separate traffic channel. The mobile unit and the base station may then exchange messages over the air interface using the traffic channel.
The base station may also initiate communication with the mobile unit. For example, the base station may receive a signal indicating that another user has placed a call destined for the mobile unit. The base station may then attempt to locate the mobile unit by broadcasting a paging message over an area associated with the last known position of the mobile unit. If the base station receives a paging response from the mobile unit, indicating that the mobile unit is in the area near the last known position, the base station may initiate a series actions and/or procedures to establish a wireless communication link with the mobile unit. However, if the base station does not receive a paging response from the mobile unit, the Radio Access Network (RAN) may escalate the level of paging (e.g., broaden the paging area) by providing a paging signal to one or more areas proximate the area near the last known position. If the mobile unit continues to be unresponsive to the paging signal, paging signals may be broadcast to additional areas until the mobile unit is located or the number of allowed pages has been exhausted. This strategy for locating mobile units is conventionally referred to as “paging escalation.”
FIG. 1 conceptually illustrates a conventional procedure 100 for paging a mobile unit (MU) and establishing a wireless communication channel. In the illustrated embodiment, a mobile switching center (MSC) initiates the call flow by providing a page request to a base station (BS), as indicated by the arrow 105. The base station provides a paging message in response to receiving the page request, as indicated by the arrow 110. If the mobile unit receives the page message, the mobile unit provides a page response message to the base station, as indicated by the arrow 115. The base station acknowledges the page response message, as indicated by the arrow 120, and provides a base station page response to the mobile switching center, as indicated by the arrow 125. The base station may provide a base station traffic channel initialization message to the mobile unit, as indicated by the arrow 130. The base station may also receive a page response message from the mobile switching center, as indicated by the arrow 135.
In the illustrated embodiment, the base station then provides a traffic channel assignment message to the mobile unit, as indicated by the arrow 140, and the mobile unit may respond with a mobile unit traffic channel initialization message, as indicated by the arrow 145. The base station may then acknowledge the mobile unit traffic channel initialization message, as indicated by the arrow 150. A backhaul link between the base station and the mobile switching center may then be set up, as indicated by the double-headed arrow 155, and a service configuration may be negotiated between the mobile unit and the base station, as indicated by the double-headed arrow 160. The base station may transmit a traffic channel confirmation message to the mobile switching center as indicated by the arrow 165. The base station may also transmit an alert message to the mobile unit, as indicated by the arrow 170. The mobile unit may use the alert message to provide an alert signal, such as a ring, a vibration, a flashing light, and the like to the end user. The mobile unit may then connect to the base station, as indicated by the arrow 175, and then the base station may provide an answer message to the mobile switching center, as indicated by the arrow 180.
Recent events have drawn attention to the possibility that the conventional techniques for paging a mobile unit may be put to malicious use. For example, alerting circuitry in a mobile unit, such as the circuitry that provides ring tones, vibration, flashing lights, and the like, may be wired to an explosive device. The mobile phone and the explosive device may be placed in a vulnerable location such as a tunnel or a bridge and the explosive device may be detonated remotely by placing a call to the mobile unit. For example, the wireless network may locate the mobile unit by transmitting a page message and may transmit an alert message in response to receiving a page response message from the mobile unit. When the mobile unit receives the alert message, an electrical signal may be provided to the alerting circuitry, which may transmit the signal to the explosive device, triggering detonation.
One proposal for preventing or reducing the possibility of an explosive device being triggered by a conventional mobile unit is to define designated security areas near vulnerable locations. Page messages to mobile units in these designated security areas may then be blocked so that a call placed to the mobile units in the designated security areas cannot be used to detonate an explosive device. However, blocking page messages to a large area can dramatically reduce the efficiency of the wireless network, at least in part because of paging escalation. In particular, if page messages to all mobile units in the designated security area are blocked, then the designated security area will appear as a coverage hole to the wireless communication network, and mobile units in the designated security area will not be visible to the wireless communication network. Accordingly, calls to the mobile units in the designated security area will result in paging escalation to a broader area as the wireless communication network searches for the mobile units that are in the designated security area.
If the designated security area is relatively small, e.g., limited to one or two cells, the network impact caused by the paging escalation may be relatively small. However, the network impact of paging escalation may be significant if the designated security area is relatively large and the designated security area covers many cells. For example, escalation of the paging area may cause a paging channel load increase on base stations neighboring the designated security area, as well as an increase in the network traffic load, an increase in the processor occupancy load, and the like. These increased loads may cause degradation of the overall system performance, which may increase the likelihood of blocking and/or call dropping.